May I advise that you not do that? There are so many image quality enhancements in Process Version 2012 that you'd be losing image quality to go back to Process Version 2010. Honestly, Fill Light does not work as well as the tools in PV 2012. Go to your favorite search engine and search for Process Version 2012 to find plenty of examples and tutorials.
I agree that you will get better results from the process 2012 settings. However, if you really want process version 2010 to be the default, highlight a newly imported image that you haven't worked with and change process version to 2010. Then, save new camera defaults. I don't think you'll be as happy with the outcome.
In case you need another opinion: Don't!
The PV2012 tools are far superior in terms of power and quality. The Fill Light slider is a very poor substitute and you would be well-served to abandon it.